Recently, there has been a growing demand for higher density recording. To meet such demand, it is known to smooth the surface of a magnetic layer. However, when a surface of a magnetic layer is made smooth, the friction coefficient between a magnetic layer and a magnetic head is increased while the magnetic recording medium is running. As a result, the magnetic layer of the magnetic recording medium tends to be damaged or to be peeled off in a short period of time.
To overcome the above problems, there have been used in recording media lubricating agents such as fatty acids, fatty acid esters, hydrocarbons, or silicone compounds, etc.
As a device for driving a flexible disk, for example, such as a home VTR (video tape recorder), a personal computer or a word processor is widely used, a magnetic recording medium comes to be driven under a variety of conditions, including low temperature, or high temperature and high humidity. To this end, a magnetic recording medium should be stable so that its running durability does not fluctuate under various expected conditions. However, conventional lubricating agents are not sufficient to overcome the above problems.
Silicone compounds have been used to solve the above problems by taking advantage of their specific ability to improve a surface property and their excellent fluidity.
As silicone compounds, JP-B-49-14249 (the term "JP-B" as used herein means an "examined Japanese patent publication") describes the following compounds represented by formula (II), and JP-A-50-32904 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes the following compounds represented by formula (III). ##STR2## (wherein R and R' each represents a hydrocarbon group having 7 to 26 carbon atoms; and n represents an integer of 1 to 20) ##STR3## (wherein R and R' each represents a hydrocarbon group having 8 to 18 carbon atoms; R" represents a hydrogen atom or a methyl group; 15.gtoreq.m.gtoreq.1; and 16.gtoreq.(n.sub.1 +n.sub.2).gtoreq.2)
However, the compounds of formula (II) are undesirable because durability is not improved, and compatibility with a binder is low, thereby causing blooming. It is thought that this is because hydrocarbon groups are attached to silicon atoms by an ether linkage having a small polarity.
The compounds of formula (III) are undesirable because the dynamic friction coefficient between the magnetic recording medium containing said compounds and the magnetic head of a VTR is increased under high loads, thereby causing creaking while the magnetic recording medium is in contact with the magnetic head. Also, the durability is hardly improved. This may be attributed to the presence of a hydrophilic alkylene oxide chain in the molecule.
To overcome the above problems, JP-B-56-26890 proposes a magnetic recording medium comprising a nonmagnetic support having thereon a magnetic layer comprising ferromagnetic particles dispersed in a binder, said magnetic layer containing an organic silicon compound represented by formula (IV): ##STR4## wherein R, R', and R" each represents a saturated or unsaturated hydrocarbon group having 7 to 21 carbon atoms; m represents an integer of 1 to 100; n represents an integer of 0 to 250; m+n.ltoreq.300; and m.gtoreq.n/5.
JP-B-56-26890 states that stable running property, good wear resistance and good durability are obtainable, and also blooming phenomenon is widely improved.
However, when such organic silicone compounds are those modified with a straight chain saturated fatty acid, melting point (softening point) increases, thereby lowering lubricating effect at low temperature. On the other hand, when such organic silicone compounds are those modified with a straight chain unsaturated fatty acid, though they have excellent fluidity, they are undesirable because static friction coefficient is increased when they are applied to magnetic recording mediums for high density recording having a smooth surface.